Applying a series capacitor in long-distance AC power transmission line is an effective method to increase power transmission capacity and improve transient stability. This has been widely adopted in Chinese large-capacity power transmission projects, such as Togtoh power plant and Yimin power plant in Inner Mongolia province, as well as Jinjie power plant in Shaanxi Province. However, the series-compensated line may lead to torsional interaction of turbine-generator shaft and cause subsynchronous resonance (SSR) problem, which is a severe threat to the generator operation security.
The academic and engineering circles have done plenty of work to solve the SSR problem, including adding auxiliary control signal to the generator excitation system as well as installing block filter between generator and series capacitor. The thyristor-based flexible AC transmission system (FACTS) can also help suppress SSR, provided that the controller is well tuned. However, the parameter tuning approaches for the above-mentioned equipment are quite complex, the suppression effect highly depends on system condition. Besides, the FACTS may inject harmonics into the AC system due to the adoption of thyristor; and its transient response is relatively slow, for the reason that thyristor is triggered once during one period.
Bypass damping filter (BDF) is a kind of filter equipment, which is usually in parallel connection with the series capacitor and is used to suppress SSR. FIG. 1 shows its structure. The parallel combination of the reactor and the capacitor in the BDF is designed to be tuned at the system rated frequency so that its impedance at the rated frequency is high. In normal operation conditions, the BDF carries virtually no currents and does not influence the steady-state operation of system. In transient states, as the impedance of the parallel combination decreases significantly at the subsynchronous frequencies, the BDF acts as a resistive/inductive bypass path for subsynchronous currents and the damping resistor in the BDF has suppression effect on the sub synchronous currents.
SSR has three categories, namely the induction generator effect, torsional interaction and torque amplification. BDF was generally considered particularly effectively for the induction generator effect. This is because at subsynchronous frequencies a synchronous generator performs like an induction generator and hence has negative resistor effect; the negative resistor can be offset to some degree by the damping resistor of BDF. However, so far the positive effect of BDF on suppressing torsional interaction and torque amplification has not been fully recognized, therefore the relevant parameter tuning approach of BDF has not been proposed.